As ideas for derivative horror films go, “The Crazies” isn’t a bad one, assuming the director approaches it with some craft and avoids cheap jump scares or heavy handed style. Luckily “The Crazies” has that in Breck Eisner (“Sahara”) who has enough of an understanding of the weakenesses inherent in this sort of thing to get around it.

After a local man wanders around town with a shotgun, Sheriff Dutton (Timothy Olyphant) starts looking for a reason why and it looks like he’s found it when he discovers the wreckage of a plane in the local swamp. The local swamp that feeds into the town’s water supply.

It’s pretty efficient as set ups go; it creates a believably motiveless villain so that the film can jump right to the mayhem. But it also allows a constant undercurrent of tension as survivors must worry if they have been infected or are just responding to the stress of their situation.

There’s nothing spectacular at work in “The Crazies” but that probably comes as no surprise. It’s got a decently slow build and just enough of a focus on the Sheriff and his wife, the town doctor (Radha Mitchell), to make you care about them and the trouble they find themselves in. Eisner understands how to build tension with skill and grace, mostly avoiding clumsy jump scares. Real horror, giving you time to understand it and squirm waiting for it to go through. Like a man locking his family in a house and methodically setting it on fire.

And in Sheriff Dutton, Olyphant has finally found a role that responds to his limited talents. Mostly he just needs earnest fear, and that’s well within his range.

There are a few decent twists and turns as well, as the U.S. Military arrives to quietly lock down the town and the unaffected townsfolk suddenly find themselves with more than just the Crazies to worry about.

It doesn’t stray that far from its conventional horror roots, though. A lot of the dramatic tension comes more from wondering if the story will jet off in a new direction or do what you expect of it. It doesn’t jet off very often.

But what it does, it does… adequately. Which, if we’re honest, is a lot more than you expect out of something called “The Crazies.”